1. Field of the Invention
This invention relates to photochromic glasses, i.e. to glass compositions which darken on exposure to actinic radiation and fade back to their original, normally colourless, state when no longer exposed.
2. Description of the Prior Art
Photochromic boro-silicate glasses have been previously described, e.g. in U.S. Pat. No. 3,208,860 (Armistead and Stockey), and are available on the market. These glasses, while exhibiting desirable photochromic properties, have relatively slow responses to exposure to, and removal of, actinic radiation, i.e. slow darkening and fading rates. It is desirable, particularly for ophthalmic purposes, to have glasses with faster responses, particularly a faster fading rate. A rapid fading rate is desirable to aid in adjusting to a sudden decrease in available light, as when a wearer of spectacles with lenses of photochromic glass enters a dimly-lit room.